1. Field of the Technology
The present disclosure is directed to rolls used for high pressure comminution of granular materials such as, for example, minerals and ores in high pressure grinding mills. More specifically, the disclosure is directed to articles adapted for use as wear resistant working surfaces of rolls and to methods of making the articles and rolls including the articles.
2. Description of the Background of the Technology
The comminution of granular materials such as, for example, minerals and ores, is often carried out between rolls in a high pressure grinding mill. High pressure grinding mills typically utilize a pair of opposed counter-rotating grinding rolls. The rotation axis of one of the grinding rolls is fixed, and the rotation axis of the second roll is floating. A hydraulic system connected to the floating roll controls the position of the floating roll relative to the fixed roll, providing pressure between the rolls and an adjustable grinding force on material passing between the rolls. The rotational speed of the rolls is also adjustable to optimize the grinding conditions. By controlling the gap between the rolls, the speed of the rolls, and the applied force, the ore or other materials passing between the rolls can be crushed in an efficient manner with relatively low energy input.
During high pressure grinding of granular materials, the material to be ground is fed into the gap between the rolls. The gap is referred to as the “nip”, and also may be referred to as the “roll gap”. The grinding of ore passing into the nip, for example, occurs by a mechanism of inter-particle breakage caused by the very high pressures developed within the material stream as it passes between the counter-rotating rolls. In addition, ore ground in this way exhibits cracks in the ore grains, which is beneficial to downstream processing of the ore.
As can be expected, the grinding operation exerts very high levels of mechanical stress on the grinding rolls of high pressure grinding apparatuses, and the grinding rolls may quickly wear.
One known approach to improve the wear resistance of a roll surface is by welding layers of hard metallic material onto the surface. FIG. 1 depicts a prior art grinding roll including a wear resistant welded surface layer. The welding process may be time consuming and expensive.
Another known approach to improve wear resistance of a grinding roll surface is by providing hard regions that project from the working surface of the roll. FIG. 2 depicts two views of a prior art roll including welded hard regions projecting from the working surface of the roll. The top view in FIG. 2 is a magnified view of the roll surface showing the individual projections and gaps between the projections. The gaps trap fine grains of the material being ground, providing autogenous wear protection to the roll surface.
U.S. Pat. Nos. 5,203,513 and 7,497,396 disclose rolls adapted for use in high pressure grinding mills and that include hard projections with gaps therebetween. As with the prior art roll depicted in FIG. 2, the gaps between the hard projections trap fine particles of the material being ground, and the particles provide autogenous wear protection to the roll surface. Also, friction between the trapped fine particles and the material being ground helps to draw the material to be ground into the nip. The method described in the '513 and '396 patents to fabricate the rolls essentially involves welding the hard projections onto the roll surface.
U.S. Pat. Nos. 6,086,003 and 5,755,033 also disclose rolls adapted for use in high pressure grinding mills that include hard projections and gaps between the projections. The method described in the '003 and '033 patents to fabricate the grinding rolls involves embedding hard bodies within a mass of metallic powder and consolidating the powder by hot isostatic pressing.
The methods for fabricating wear resistant high pressure rolls described in the above-identified patents are costly and tedious. For example, the use of a welding process to secure hard elements to a roll surface limits the range of materials from which the hard elements can be fabricated. Hot isostatic pressing of a large roll requires the use of expensive equipment, and a grinding roll fabricated by hot isostatic pressing cannot be repaired easily in the field.
Accordingly, there is a need for articles and methods improving the wear resistance of the working surface of grinding rolls. It is desirable that such articles and methods require relatively inexpensive equipment; allow a wide range of materials to be used as the projecting hard elements; permit tailoring of the base material used in the grinding roll; and permit easy repair of the roll surface in the field.